This invention is related to flow meter instrumentation. More particularly, the invention is related to obstruction flow meters which are used in series in a flow conduit to determine the volume flow rate of liquid and gas phases of fluid flowing within the conduit.
Fluid flow meters are used in many areas of industry and commerce. Various nuclear, acoustic, electromagnetic and mechanical techniques have been used to measure flow rate and volume flow rates of fluids containing one, two, or more components or xe2x80x9cphasesxe2x80x9d.
Obstruction type flow meters are widely used to measure single phase flow, such as fluids comprising 100% liquid or 100% gas. In orifice flow meters, fluid is forced to flow through an orifice in a plate within the flow conduit, creating a pressure drop across the plate. Orifice flow meters are relatively inexpensive to fabricate and maintain, and are reliable in many types of field operations. In addition, the physical size of most orifice devices is relatively small. Measurements of the differential pressure across the plate, along with fluid pressure and temperature measurements, are used to compute flow rate using equations well known in the art. Orifice flow measurements can be used to measure multiple-phase flow only if an independent measure of the ratio of the phases is made. Furthermore, accurate measurement of the volume flow rates of each phase can be obtained only if the linear flow velocities of the phases are the same, or the relative velocities or xe2x80x9cslippagexe2x80x9d of the linear phase flows can be determined, or all phases are forced to flow at the same linear flow rate at the position which the phase ratio and orifice plate measurements are made.
Positive displacement type flow meters force fluid to flow through a positive displacement meter such as a turbine apparatus, and the flow rate of the fluid is determined from the rate of revolution of the flow meter turbine. Positive displacement type flow meters may be used in multiple-phase flows. As with orifice flow meters, independent phase ratio measurements must be made using a variety of technologies, and assumptions must be made concerning the linear flow velocities of each of the phases in order to obtain accurate volume flow rates for the individual phases. Positive displacement type flow meters are more complex, more costly to manufacture and maintain, and are generally larger than orifice flow meters.
Separators are widely used in multiple-phase flow measurements. As an example, in the petroleum industry, it is of interest to measure volume flow rates of the three fluids produced: oil, gas and water. Gravity separators are widely used to separate these three components. The separated components are then drawn from the separator and single phase flow measurements are made on each of the separated components. Characteristically, separators are physically large, are expensive to construct, require a relatively long period of time for the multiple phases to separate by means of the force of gravity, and require separate flow meters and flow controllers for each separated phase.
Various two and three-phase xe2x80x9cin-linexe2x80x9d flow meters have been developed, especially in the petroleum industry. Relatively accurate three-phase xe2x80x9cpartitionxe2x80x9d measurements can be made using nuclear, acoustic, electromagnetic, and/or a combination of these technologies. However, a problem lies in accurately determining the flow velocities of each of the phases. Various relationships have been developed to calculate the relative or xe2x80x9cslippagexe2x80x9d velocity of two phases with respect to a measured third phase, but the calculations are replete with assumptions. In addition, these devices are usually quite complex both electronically and mechanically, are expensive to fabricate, and very expensive to maintain and calibrate.
Significant progress has been made recently in the area of single plate obstruction flow meters. U.S. Pat. No. 5,295,397 issued to Hall et al. on Mar. 22, 1994, and entitled xe2x80x9cSlotted Orifice Flowmeterxe2x80x9d (""397) discloses an orifice flow meter. The orifice plate is designed such that measurements are relatively insensitive to upstream and downstream flow conditions. In addition, this orifice plate is less disruptive in the manner in which it is used to impede flow. Therefore, fluid pressure recovers more readily within a shorter distance from the flow meter, and incurs less unrecoverable pressure drop than prior art orifice flow meters. Independent phase ratio measurements must be made, or assumptions directed toward the multiple phases must be made, in order to use the ""397 device to measure volume flow rates in multiple-phase fluid flows. This patent is incorporated herein by reference.
U.S. Pat. No. 5,461,932 issued to Hall et al. on Oct. 31, 1995, and entitled xe2x80x9cSlotted Orifice Flowmeterxe2x80x9d (""932) discloses an orifice flow meter. A phase ratio sensor is used upstream from the orifice plate to allow two-phase flow measurements to be made without necessitating separation of the fluid. However, the phase ratio measurement is completely separate from the orifice flow meter measurement. This patent is incorporated herein by reference.
In one aspect of the present invention, a flow meter is provided for measuring the flow rate of each phase of a multiple-phase fluid in a conduit. Obstruction flow meters are serially positioned in a conduit and spaced a predetermined distance apart based upon the configuration of the orifice flow plate used in the obstruction flow meters. Sensors are also positioned in the conduit to measure the pressure and temperature of the multiple-phase fluid at various locations relative to the obstruction flow meters. The measurements are fed to a computer which calculates the flow rate of each phase of the multiple-phase fluid.
In another aspect of the present invention, a method is provided for measuring the flow rate of each phase of a multiple-phase fluid in a conduit. Obstruction flow meters are serially positioned in a conduit to create flow impedances. Pressures and the temperature of the multiple-phase fluid are measured at various locations relative to the obstruction flow meters. The measurements are then used to generate the flow rates of each phase of the multiple-phase fluid.
In another aspect of the present invention, a flow meter is provided for measuring a mixture of offshore petroleum products flowing in a conduit. Three obstruction flow meters are serially positioned in a conduit and spaced a predetermined distance apart based upon the configuration of the orifice flow plate used in the three obstruction flow meters. Sensors are also positioned in the conduit to measure the pressure and temperature of the mixture at various locations relative to the three obstruction flow meters. The measurements are fed to a computer which calculates the flow rate of each phase of the mixture. The flow rates are then stored in a memory device for future reference when determining royalty payments.
A primary technical advantage of the present invention is to provide multiple-phase flow measurements without the use of an independent phase ratio measurement.
Another primary technical advantage of the present invention is to provide a flow meter and a method for calculating more accurate values of the Reynolds number of the fluid and the xe2x80x9cqualityxe2x80x9d of the gas from pressure, temperature and differential temperature measurements made in the vicinity of the obstruction flow meters.
An additional technical advantage of the present invention is to provide a reliable, relatively inexpensive, compact means for measuring multiple-phase flow which is compatible with instrumentation of single-phase orifice flow meters, thereby eliminating the necessity to employ exotic and/or expensive technologies such as sonic, nuclear, electromagnetic imaging, phase separation and the like to obtain multiple-phase measurements.
A still further technical advantage of the present invention is to provide a multiple-phase flow meter for offshore petroleum production operations where space on drilling and production platforms is at a premium, and reliability is of paramount importance.
Further advantages of the present invention may be appreciated upon examining the specification and claims below.